Ion implantation is one of the key processes in the fabrication of semiconductor devices. Dopant ions such as boron, phosphorus or arsenic are created from a high purity gas source and implanted in a semiconductor substrate. Each doped atom creates a charge carrier, either hole or electron and thus modifies the conductivity of the semiconductor device in its vicinity. Ion implantation is commonly applied to the source/drain junction and the channel to achieve desired electrical characteristics of the devices to be produced.
In a typical ion implantation process, a substrate (e.g., silicon wafer) is first subjected to an organic chemical pre-treatment and then a positive-tone photoresist is coated on the substrate. After hot baking, edge bead removal, exposure, development and spin-drying steps, an organic photoresist mask is formed. During ion implantation process, dopants penetrate into the exposed (unmasked) surface of the substrate as well as the photoresist mask. The dopants may react with the photoresist mask to form a relatively nonporous layer, which is commonly known as a “crust.” After completion of the ion implantation process, the photoresist mask is then removed by a stripping process. Typical post-ion implantation stripping is done by a dry plasma ashing followed by a wet piranha clean (which uses a mixture of sulfuric acid and hydrogen peroxide as clean agents) and a marangoni dry. Although the above process is widely used in the semiconductor industry, some drawbacks such as long process time and damage to silicon substrates have been noted. Silicon substrate damage such as silicon loss has become a key issue as the critical dimension shrinks to 45 nm and below. Silicon loss of greater than 30 Å may result in undesirable dopant out diffusion and cause device malfunction. For these reasons, the typical process for post-ion implantation stripping process is no longer acceptable and there is need for a new process.
Various methods for removal of the photoresist after ion implantation process are discussed in prior art. For example, U.S. Pat. No. 6,524,936 entitled to Hallock et al. discloses a method which exposes a wafer under UV radiation of 200 nm to 400 nm and at least 100 mJ/cm2 prior to conventional wet or dry stripping processes. In U.S. Pat. No. 5,811,358 entitled to Tseng et al., a three-step procedure is disclosed. The substrate is first stripped with an oxygen and nitrogen/hydrogen plasma at a low temperature (<220° C.) to minimize the photoresist solvent popping problem. Then, a higher temperature (>220° C.) is employed to remove the remaining photoresist. Finally, the substrate is cleaned with ammonium hydroxide and hydrogen peroxide mixtures. Nevertheless, the abovementioned approaches still suffer from unacceptable silicon loss.
Photoresist stripping compositions are disclosed in numerous prior art. For example, U.S. Pat. No. 6,551,973 entitled to Moore discloses a stripping composition comprising benzyl-trimethylammonium hydroxide (BTMAH) and a solvent system comprising alkylsulfoxide and optionally a glycol co-solvent, corrosion inhibitor and non-ionic surfactant for removing polymeric organic substances from metalized inorganic substrates. In U.S. Publication No. 2007/0099805 to Phenis et al., a stripper solution comprising dimethyl sulfoxide and a quaternary ammonium hydroxide and an alkanolamine is disclosed. However, attempts to employ conventional stripping compositions to remove a photoresist after ion implantation, especially heavy dose ion implantation, have always failed because the photoresist becomes nonporous and forms a crust after ion implantation. The nonporous crust prevents the penetration of the wet chemicals into the inner portion of the photoresist and thus significantly reduces the contact area between the wet chemicals and photoresist. In addition, the crust portion is highly non-uniform and thus the process difficulty of a wet clean is increased. Accordingly, post-ion implantation stripping by conventional wet chemicals is impractical.